1. Field of the Invention
This invention relates to laser shock processing in which an absorbing material layer for absorbing laser light is first provided on the surface of a metallic workpiece and subsequently, the absorbing material layer is covered with a light transmitting member layer, and thereafter, these layers are irradiated with laser light pulses, whereby a shock due to the evaporation of the absorbing material is passed to the metallic workpiece.
2. Description of the Prior Art
Previously, a method of applying a shock to a metallic material and increasing the compressive residual stress thereof has been performed for the purpose of improving the physical properties such as the mechanical strength. An example of such a method is what is called a laser shock processing method which can apply a large shock to a metallic material locally and thus has been put to various uses.
For example, an example of the conventional laser shock processing method is disclosed in the Japanese Patent Public Disclosure Official Gazette (Kokai Koho) No. 58-120716/1983 (JP-A-58120716) which corresponds to U.S. patent application Ser. No. 334,612. FIG. 16 is a diagram illustrating this conventional laser shock processing method. As shown in FIG. 16, the top surface 41a and the bottom surface 41b of a metallic target 41 are coated with absorbing coating materials (coatings or paints (not shown)). A first overlay (a light transmitting member) 42 is mounted on the coated top surface 41a of the target 41 and a second overlay (a light transmitting member) 43 is mounted on the coated bottom surface 41b thereof.
A high-energy laser light short pulse 51 emitted from a laser 44 is split by a spectroscope (a semitransparent mirror) 45 into laser light pulses 52 and 53. The laser light pulse 52 is sequentially reflected by a first mirror 46 and a second mirror 47 in this order. Then, the reflected laser light pulse is focused by a first convex lens 48 and is further transmitted by the first overlay 42. The coating formed on the top surface 41a of the target 41 is irradiated with the transmitted laser light pulse. On the other hand, the laser light pulse 53 is reflected by a third mirror 49 and is then focused by a second convex lens 50. Subsequently, the focused laser light pulse 53 is transmitted by the second overlay 43. Thereafter, the coating formed on the bottom surface 41b of the target 41 is irradiated with the transmitted laser light pulse 53.
When irradiating the coatings with the laser light pulses 52 and 53, evaporation coating gas is produced from the surfaces of the coatings and further expands instantaneously. Then, the pressure exerted on the top surface 41a and the bottom surface 41b of the target 41 increases almost instantaneously owing to the presence of the first overlay 42 and the second overlay 43. This results in that the shock wave of pressure is applied to the top surface 41a and the bottom surface 41b of the target 41. This shock wave causes compressive residual stress in the surface portion of the target 41. Moreover, the fatigue strength of the target 41 increases owing to this compressive residual stress.
Thus, in accordance with this conventional method, compressive residual stress can be imparted to a desired portion of the metallic target 41. Therefore, this conventional method is suitable for increasing the fatigue strength of a bent portion of a crankshaft, which is locally strained.
However, this laser shock processing technique is comparatively new. Thus, only a small quantity of data regarding actual results of this processing has been accumulated. Further, various experiments performed by employing this conventional technique have showed that there have been many cases where the fatigue strength is not sufficiently increased.
Thus, extensive studies of this laser shock processing have been further conducted. As a result, it has come to light that if the coatings formed on the top surface 41a and the bottom surface 41b of the target 41 do not have even thickness, nonuniform compressive residual stress is caused therein and that if the compressive residual stress is insufficient in a part of the target 41, this part of the target 41 does not have sufficient fatigue strength.
Especially, in case where the same portion of the surface of the target 41 is irradiated with laser light pulses many times in order to exert as deep an effect on the target 41 as possible, and in case where a large area of the surface of the target 41 is continuously processed by performing partially overlapping irradiations of laser light pulses, the surface of the target 41 is recoated with the coating or paint prior to each of the irradiations. It has been discovered that in such cases, not all of the coating applied to the surface of the target 41 is evaporated at a laser light irradiation, that it is, therefore, difficult to control the thickness of the coating in such a manner as to be uniform before each irradiation of the laser light pulse and that the coating is thus liable to have non-uniform thickness.
Further, in the case of employing the conventional method, after the surface of the target is coated with the absorbing coating materials and the overlay 42 or 43, the thickness of a film consisting of the absorbing coating material and the overlay 42 or 43 can not be held constant. As a result, the focal distance of the lens changes with every irradiation of the laser light pulse. Consequently, the impartation of uniform residual stress to the target 41 can not be realized.
Moreover, in the case where the surface of the target is coated with the coating or paint, the step of drying the coating is necessary. Thus, there has been a demand for omission of the drying step. Especially, in the case of repeating the laser shock processing, the omission of the drying step greatly facilitates the laser shock processing.